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A major concern of group housing is aggression at mixing, which impairs both welfare and performance of sows. Aggression is associated with excitement, and suppressing excitement at mixing reduces aggression. Tryptophan (Trp), a precursor of serotonin, reduces aggression in humans and animals. The objective of this study was to determine effects of dietary Trp supplementation on mixing-induced aggression and the associated stress, and on reproductive performance in gestating sows. Sows were mixed after weaning in pens with electronic sow feeders on concrete-slatted floors. Each pen housed 21 ± 2.8 sows, with approximately 2.7 ± 0.43 m2/sow of floor space allowance. Multiparous sows (n = 168) from 8 breeding groups were used, with 4 groups assigned to the control diet, and 4 groups assigned to the high-Trp diet. Control sows received corn-soybean meal based diets throughout gestation (0.15% Trp) and lactation (0.21% Trp). Sows assigned to high-Trp treatment received about 2.3 times the dietary Trp (0.35% in gestation diet and 0.48% in lactation diet) fed to control sows 3 d before and after mixing. Six focal sows (2 sows from each of parity 1, 2, and 3 or greater) in each pen were designated and videotaped after mixing for 72 h to determine the type and number of aggressive interactions among sows. Before and 48 h after mixing, saliva samples were collected from focal sows, and scratches were assessed on all sows.
Aggression among sows was intense during the initial 6 h, and decreased between 6 and 72 h after mixing. The initial aggression caused scratches and elevated cortisol concentrations (P < 0.05). Mature sows tended to fight for longer periods (112 vs. 52 s•h-1•sow-1, SE = 23.8; P < 0.10), but had fewer scratches caused by aggression (injury score = 4.3 vs. 6.5, SE = 1.13; P < 0.01) than parity-1 sows. Supplementation of dietary Trp reduced the total duration of head-to-head knocking (P < 0.05), but did not affect other aggressive behaviors. There was no difference in injury scores or saliva cortisol concentrations between dietary treatments. High-Trp sows had more total born (12.5 vs. 10.5 pigs/litter, SE = 0.55; P < 0.05) and still born piglets (1.5 vs. 0.8 pigs/litter, SE = 0.20; P < 0.05), but had no significant change in born alive piglets (10.8 vs. 9.7 pigs/litter, SE = 0.42) compared with control sows.
In conclusion, the results indicate that the initial aggression after mixing caused more injuries in young sows than in mature sows. Supplementation of dietary Trp at 2.3 times the control level for a short period did not effectively reduce aggression and the associated stress in sows at mixing.
YZ Li, SK Baidoo, LJ Johnston and JE Anderson, 2011. Journal of Animal Science, 89(6): 1899-1907. http://dx.doi.org/10.2527/jas.2010-3125
